Recently, several interferometeric techniques have been developed to measure ultrasonic, photo acoustic and photo thermal displacement in non-destructive testing. The ability to focus a very small area gives high spatial resolution. Generally two types of interferometeric techniques have been used in ultrasonic and photo thermal displacement detection. One such technique is the classical Michelson interferometer, and a second is based on a doppler shift of the frequency of the reflected or scattered light, caused by surface oscillations and its demodulation by an interferometer. This scheme is sensitive to surface velocity.
The Michelson interferometer is very common in intereferometry and disclosed in R.S. Sharpe, ed. Research Techniques in NDT, vol. VII, p. 267. The incident laser beam is split into two components by a beam splitter The first component is deflected by 90 degrees by the beam splitter constituting the reference beam. This reference beam is reflected from a plane mirror and retraces its path to the beam splitter and passes undeflected therethrough. The second component passes through the beam splitter undeflected and constitutes the probe beam. This probe beam is reflected off the sample surface and retraces the path back to the beam splitter, where it is deflected by 90 degrees. The two reflected beams are recombined at the beam splitter and travel together to the detector. Since both beams originate from the same coherent source, interference fringes are formed at the detector which are determined by the optical path difference of the two beams. Each fringe corresponds to a displacement of the quarter of the wave length of light. The fringe intensity varies sinusoidally as a function of optical path difference. The Michelson interferometer is not suitable to measure displacements much less than a wave length The noise introduced by mechanical and thermal fluctuations causes the relative path length to fluctuate It is generally very sensitive to air currents and vibrations.
Another interferometeric technique which detects ultrasonic motion uses wavelength transmission selectivity of the Fabry-Perot interferometer to detect the wavelength changes due to Doppler shift. The sample surface is illuminated by a frequency-stablized laser and the reflected and scattered light is collected by a telephoto lens. This light is received by a confocal Fabry-Perot interferometer which optimizes the light gathering power and illuminates only the central fringe. The change in wavelength is proportional to the surface displacement velocity. A Wollaston prism is used to split the beam. This makes both the reference and probe beam equal One of the two beams is focused onto an undisturbed portion of the sample and serves as the reference The other beam is focused onto a position of the sample surface periodically heated. The surface displacement due to thermal expansion causes phase modulation of this beam. Both beams reflected from the sample recombine in the Wollaston prism and interfere on a photo-detector. J.T. Fanton and G.S. Kino, Appl. Phy. Lett. 51, p. 66 (1987).
Other non-interferometric systems have been developed to detect the "thermal bump". In this scheme, the sample is heated by a focused modulated pump beam, creating a periodic displacement "thermal bump" due to thermal expansion. A focused probe beam is reflected off the bump. The deflection of the reflected probe beam, which is proportional to the slope of the bump is measured. The heating beam is obtained from an Ar+ ion laser, which is intensity modulated using an accoustic-optical modulator. The beam is spatially filtered and expanded using a beam expander. Then it is reflected off a plane front surface mirror, which is mounted on a rotating stage and focused onto the sample using a microscope objective. Since the mirror is mounted off axis on the stage, the angle of incidence of the beam on the focusing lens is changed without moving away from the lens. The heated spot and hence the bump, can be scanned relative to the probe beam spot by turning the rotating stage. The probe beam is obtained from an He-Ne laser, which is spatially filtered and expanded using a beam expander. The axis of polarization of the laser is rotated by turning the laser head, in order for the beam to pass through the polarizing beam splitter It passes through a quarter wave plate and reflects off a dichroic mirror onto the same microscope objective, which focuses both beams on the sample. The probe beam is reflected from the sample, passes through the same path and arrives at the beam spliter with the axis of polarization rotated by 90 degrees. The signal is obtained from electrodes at each edge of the square cell. The deflection of the beam in two perpendicular directions is measured by taking the difference signals electronically, from the electrodes on either side. The signal is detected synchronously with the modulation of the bump using a lock-in amplifier. The optical reflectively change is also measured by taking the sum of all four electrodes. The problem with this type of technique is that it is also very sensitive to air currents and vibration. Such system is disclosed by A. Rosencwaig, J. Opsal and D.L. Willenborg, Applied Physics Lett. 43, 166 (1983).